Television system



Sept- 14, 1937. R. l.. CAMPBELL TELEVISION SYSTEM Original Filed Jan. 28, 1933 Patented Sept. 14, 1937 UNITED STATES TELEVISION SYSTEM Richard L. Campbell, Oaklyn, N. J., assignor to Radio Corporation of America, New York, N. Y., a. corporation of Delaware Application January 2s, 1933, Serial No. 653,976 Renewed October 16, 1936 9 Claims.

My invention relates to improvements in television systems.

In communication by television, it has been found advantageous to amplify and transmit the picture signals and synchronizing impulses in the same channels. This method, when using a cathode ray tube for developing the picture signals, gives rise to the following difficulty: During operating action of the cathode ray tube, the 10 ray of electrons is deflected to cause the same to scan mosaic screen structure along parallel lines. Usually, for this purpose, the ray is deected horizontally at a relatively high frequency, and is simultaneously deflected vertically at a relatively low frequency according to the number of frames per second desired. During each deflection of the ray from left to right, for example, individual electrostatic charges previously stored on the screen by light from the subject being transmitted, are neutralized. This action causes development of picture current in the output circuit of the tube, representative of conditions of.

light and shade at the individual elemental areas of the subject. The ray is returned at a much greater rate from right to left, and Iduring this time is generally cut olf and of no effect with respect to the screen or any part of the circuit in the way of developing picture signals. It has been found, however, that during this return deflection of the ray, it still has some influence on the screen to develop undesirable electrical waves or impulses Which, when amplified and transmitted with the picture signals and the synchronizing'impulses, interfere with operating action at the receiving station.

At any rate, even assuming that the ray is completely cut on during both the horizontal and vertical return-line periods, the very action of cutting off the ray for these periods, and then releasing it to full intensity for the scanningline periods, causes development of the undesirable electrical Waves or impulses. Although these interfering Waves occur only during the periods of return deflection of the ray, and 45` simultaneously with the synchronizing impulses, they are of such nature and magnitude that, when they are received and applied to the control grid of a cathode ray tube at the receiving station, they cause the background of the reproduced picture to be non-uniform.

With the foregoing in mind, it is one of the objects of my invention to provide an improved television system and method of operation whereby, in using a cathode ray tube at the transmitting station for developing picture signais, both these signals and the synchronizing impulses developed independently can be amplified and transmitted :in the same channel to the exclusion of the interfering Waves referred to and which would otherwise be amplified and transmitted.

, Other objects and advantages will hereinafter appear.

In accordance with my invention, during spaced periods picture signals are developed representative of conditions of light and "shade at the elemental areas of the transmitted subject. Operating action is accompanied by the development of interfering electrical Waves during the periods intervening the spaced picture periods. The picture signals are amplified, to the exclusion of the interfering waves, by electrically superimposing on said Waves electrical impulses each of such'amplitude and polarity as to place each wave beyond a given level and beyond the level of the peak-picture signals representative of conditions of`shade at lthe subject. The picture signals and the superimposed impulses and the interfering waves are then supplied to an amplifying channel adjusted for cut-off at substantially said given level.

More particularly, in accordance with my invention, wherein a cathode ray tube is used to de velop picture signals during spaced periods and which are representative of conditons of light and shade at the individual elemental areas of the transmitted subject, synchronizing impulses are developed to occur only during the periods of l return deflection of the ray intervening the spaced picture-signal periods. Only the picture signals and the accompanying interfering Waves developed during horizontal return deflection of the ray are amplified in a first amplifying channel. Only the synchronizing impulses are amplied in a second amplifying channel and to a level substantially greater than that of the amplified peak-picture signals. The amplified picture signals and the interfering waves and the amplified synchronizing impulses are then supplied to a common amplifying and transmitting channel under conditions whereat the synchronizing impulses are of the same polarity as the picture signals representative of conditions of shade at the subject. The input circuit of the common amplifying and transmitting channel is effective to cut off passage through the same of all voltages ofsaid polarity and whose amplitude is somewhat above the level of the peak-picture signals of said polarity. In other words, the individual Waves are caused to appear on the tips of the amplified synchronizingV impulses, and in starting through the common amplifying and transmitting channel, the tips or peaks of these impulses which are then distorted by the interfering waves referred to, are saturated off, so to speak. After this action, the synchroniliing impulsesA have a substantially squaretopand are then amplified and transmitted without distortion which would otherwise interfere with reproduction of the picture at the receiving station.

My invention resides in the improved system, apparatus and method of operation of the character hereinafter described vand. claimed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawing, wherein Figure 1 is a simpliled, diagrammatic view of a television transmission system embodying my invention; and

Figs. 1a, 1b, 1c and 1d are schematic figures, illustrative of the principle of operation of the system shown in Fig. 1.

With reference to Fig. 1, the reference numeral I designates a cathode ray tube for developing picture signals. This tube comprises suitable.

photosensitive, mosaic screen structure I2 which, for example, may be made up of a great number of minute, photosensitized silver globules supported on a metallic base plate and each insulated from the latter and from each other. A suitable electron gun i4 operates to develop a ray I6 of electrons and to direct the same at the screen structure. The tube is provided with a grounded anode in the form of a silver coating I'I on the inside surface. 'I'he ray I6 is caused to scan a rectangular area or frame of the screen structure twenty-four times a second. lDuring relatively short periods between the frames, the ray is returned to the initial starting point for the next succeeding frame.

Let it be assumed, for example, that there are to be 24 pictures or frames a second, each having 180 lines horizontally. For these conditions, the ray is defiected horizontally by coils I8 through which a saw-tooth current wave is cause to pass at a frequency of 4320 cycles. A suitable generator 20 supplies the current wave for this purpose. Also, the ray is deiiected vertically by coils 22 through which a saw-toothcurrent wave is caused to pass at a frequency of 24 cycles. A suitable generator 24 supplies the current wave for this purpose.

'I'he electron gun I4 is provided with a control electrode or grid 26 by which the intensity of the ray is varied in the manner and for the purpose hereinafter explained.

An image of the object 28, which may be an outdoor view, a studio scene, or a moving-picture film, is projected onto the screen I2 by a suitable lens system 30.

Looking toward the right in Fig. 1, as the ray I6 scans the screen from left to right, individual electrostatic charges, previously stored on the surface of the screen, are neutralized to develop picture signals in the output circuit of the tube I0. These signals are amplified in a suitable amplifier 40 and supplied by a connection 42 to the grid circuit of a tube 44. The amplifier 40 and the tube 44 can be considered as comprising a channelA in which the picture signals are amplified. The number of tubes in the channel A is such that, in the output circuit of the tube 44 or, what is the same, in the input circuit of the tube 46 supplied from the tube 4l4, the polarity of the picture signals representative of conditions of shade at the subject is negative. for this will hereinafter more fully appear.

` For the purpose of maintaining operation of the generators 20 and 24 at the respective frequencies at 4320 cycles and 24 cycles, a generator The reason 48 operates to develop synchronizing impulses lI! at the rate of 4320 a second, and to develop framing impulses 52 at the rate of 24 a second. These impulses are of the same amplitude, but the framing impulses 52 are of greater duration than the synchronizing impulses, as indicated. The synchronizing impulses 50, after passing through a suitable amplifier 54, are supplied to the generator 20, as indicated, and operate to drive the same at the frequency of 4320 cycles. Also, the framing impulses 52, after amplification in the amplifier 54, are supplied to the generator 24, as indicated, and operate to drive the same at the required frequency of 24 cycles. The adjustment of the generator 24 is such that it does not respond to the impulses A50 vior horizontal synchronizing.

It will be understood that picture signals are only developed as the ray is defiected horizontally from left to right and vertically from top to bottom. During return deflection of the ray from right to left, horizontally, and also during return deflection of the ray vertically from the lower right-hand corner of the screen to the upper left-hand corner, the ray serves no useful purpose in the way of developing picture signals. It is therefore desirable to cut off the ray during return deflection thereof in both horizontal and vertical directions, and for this purpose the synchronizing and. framing impulses are taken from a suitable stage of the amplifier 54 and applied by a connection 56 to the control grid 26. The polarity and amplitude of each of these impulses is such as to impart a negative bias to the grid 26 sufficient to substantially cut off the ray.

The operation of my improved system will now be explained. During each horizontal deflection of the ray I6, the stored charges on the screen I2 are neutralized to develop picture signals 58, as represented in Fig. 1b. Each of these deections of the ray occupies the time period t1. Return defiection of the ray horizontally occupies the time period t2, and it is during these return periods, intervening the spaced picture periods ti, that the undesirable electrical waves 60 are developed. Only the picture signals and the accompanying interfering waves 60 are amplified in the channel A. In the output circuit of the last stage 44 in this channel, these signals and Waves appear as in Fig. 1c, the polarity of the picture signals representative of conditions of shade at the subject 28 being negative. .The negative peak-picture signals or, in other words, the picture signals representative of a black area at the subject, are at the level 62, in the negative direction.

The synchronizing impulses 50 have a negative polarity in the last stage of the amplifier 54, as represented in Fig. 1d., and this amplifier can be considered as comprising a channel B in which only the impulses 58 and 52 are amplified. The amplified synchronizing impulses 50 are amplified to the level 64 which is substantially greater than the level 62. n Therefore, when these impulses are appliedto the grid or input circuit of the tube 46 by way of the connection 66, they are superimposed upon the interfering waves 60 and operate to carry the latter beyond the level 62 in the negative direction, as represented in Fig. 1c. The tube 46 is biased to out off at a level 68, somewhat beyond the level 62 of the negative peak-picture signals, and at the right point to prevent passage through the tube of voltages as high as any voltage of the waves 60. From the point z, the amplified picture signals and synchronizing impulses pass through a common amplifying and transmitting channel C comprised of the tubes 46 and 'l0 and a suitable radio transmitter 12. Since the'tube 46 is biased to cut-off at the level 68, asl explained, the picture signals and synchronizing impulses appear in the output circuit of this tube as represented in Fig. 1d, the synchronizing impulses 50 having substantially fiat tops and being of the same polarity as the picture signals representative of conditions of shade at the subject 28. From this it will be seen that the picture signals and the synchronizing impulses are amplified and transmitted in the channel C to the exclusion of the interfering waves 60 which would otherwise result in non-uniformity of background at the receivingstation.

In the practical embodiment of my invention,

satisfactory results have been obtained by using, for the tubes 44, 46 and 10, those identified in the trade by the designations shown. The various values of resistance given have been found to be satisfactory. It will be understood, however, that these values are not critical, and may be varied over a Wide range, to suit particular requirements. While but one embodiment Aof my invention has been disclosed, it is seen that modifications, within the conception of those skilled in the art, are possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In a television transmission system, scanning apparatus for developing picture signals during spaced periods, means operating to maintain scanning action of said apparatus at a definite rate and developing for such purpose synchronizing impulses occurring at said rate and only during the periods intervening the picture-signal periods, a first channel amplifying only the picture signals and having a final amplifying stage' in the output circuit of which the picture signals representative of conditions of shade at the transmitted subject are of negative polarity, a second channel amplifying only the synchronizing impulses and having a final amplifying stage in the output circuit of `which said impulses are of negative polarity and substantially greater in amplitude than the negative peak-picture signals in the output circuit of said first channel, a transmission channel common to said first and second amplifying channels and comprising an electron tube comprising a rst stage and whose input circuit is supplied from the output circuits of said amplifying channels, and means associated with said input circuit for biasing said tube whereby voltages supplied to said input circuit, and which are substantially more negative than the negative peak-picture signals supplied to said input circuit, are effective to cause said tube to cut oii.

2. In the art of television, the method of operation which comprises projecting an image of the subject for transmission onto photosensitive screen structure, scanning said structure during spaced periods to develop the picture signals representative respectively of conditions of light and shade at the respective elemental areas of the impulses only during the periods intervening the spaced picture periods, amplifying said impulses in a second amplifying channel and toan amplitude substantially greater than the amplitude of the peak-picture signals in the output circuit of the first amplifying channel, supplying the amplified picture signals and the amplified impulses to the input circuit of an electron tube in a common amplifying and transmitting channel under conditions whereat said impulses are of the `same polarity as the picture signals representative of conditions of shade at the subject, and biasing said tube whereby voltages of said polarity supplied to said input circuit, and which are greater by a predetermined amount than the amplitude of the peak-picture signals of the said polarity, are effective to cause the said tube to cut off.

IZi. In a television transmission system, means comprising a cathode ray tube for developing picture signals during spaced periods, a first channel for amplifying said signals to a given level,

means for developing synchronizing impulses w during the periods intervening the spaced picture periods, a second channel for amplifying said Vimpulses to a level higher than said given level, an amplifying and transmitting channel comprising an electron tube constituting the first stage thereof, the input circuit of said tube being supplied from said first and second channels under conditions whereat said impulses are of the same polarity as the picture signals representative of conditions of shade at the subject for transmission, and means associated with said input circuit of said electron tube for biasing the latter for cut-oiiat a level between said first and second-named levels.

4. In the art of television transmission, the method of operation which comprises developing during spaced periods picture signals representative of conditions of light and shade at the individual elemental areas of the transmitted subject and developing incidentally to such operation, and during the periods intervening said first-named periods, accompanying interfering electrical waves, electrically superimposing on said waves electrical impulses each of such amplitude and polarity as to place each wave beyond a given level and beyond the level of the peakpicture signals representative of conditions of shade at said subject, and supplying said picture signals and the superimposed impulses and interiering waves to an amplifying channel adjusted for cut-ofi at substantially said given level Whereby the picture signals are amplified to the exclusion of said Waves.

5. In the art of television transmission, the method of operation which comprises developing during spaced periods picture signals representative of conditions of light and shade at the individual elemental areas of the. transmitted subject and developing incidentally to such operation, and during the periods intervening the spaced picture-signal periods, accompanying interfering electrical waves, developing synchronizing impulses to occur only during said intervening periods, amplifying only the picture signals and the accompanying interfering waves in a rst amplifying channel, amplifying only the synchronizing impulses in a second amplifying channel and to a level substantially greater than that of the amplified peak-picture signals, supplying the amplified picture signals and interfering waves and the amplified synchronizing impulses to a common amplifying and transmitting channel under conditions whereat the synchronizng impulses are of the same polarity as the picture signals representative of conditions of shade at said subject, and utilizing the common amplifying channel to cut oi passage of all voltages of said polarity and Whose amplitude is above the level of the peak-picture signals of said polarity.

6. In the art of television, the method of operation which comprises developing picture signals during spaced periods and developing incidentally to such operation, and duringV the periods intervening the picture-signal periods, accompanying undesirable electrical waves, developing synchronizing impulses during said intervening periods, amplifying said signals and waves in a. rst amplifying channel, amplifying said impulses in a second amplifying channel, supplying the amplified signals and waves and impulses to the input circuit of a common amplifying and transmitting channel, and utilizing said impulses at said circuit to drive said waves beyond the operating range of the common amplifying channel.

7. In the art of television transmission, the method of operation which comprises developing during spaced periods picture signals representative of conditions of light and shade at the individual elemental areas of the transmitted subject and developing incidentally to such operation, and during the periods intervening said first-named periods, accompanying interfering electrical waves, electrically superimposing on said interfering electrical waves electrical impulses each of such amplitude and polarity as to place each wave beyond a given level and beyond the level of the peak-picture signals, and supplying said picture signals and the superimposed impulses and interfering waves to an amplifying channel and adjusted for cut-off substantially at said given level whereby the picture signals are amplified to the exclusion of said waves.

8. In a picture transmission system, means comprising a cathode ray tube for developing plcture signals during spaced periods and for developing incidentally during the periods intervening said first-named periods accompanying interfering electrical waves, means for producing electrical impulses which occur during said intervening periods, means for superimposing said interfering waves and said impulses, said impulses having an amplitude suflicient to place said interfering waves beyond a given level which is beyond the level of the peak-picture signals, an amplifier channel so adjusted that it does not transmit beyond substantially said given level, and means for so supplying said superimposed waves and impulses to said channel that the picture signals are amplied to the exclusion of said interferingwaves.

9. In a picture transmission system, an electron discharge tube including means for forming an electrical image, means for scanning said image during spaced periods whereby picture signals are developed and whereby interfering signals are developed incidentally during the periods intervening said first-named periods, means for producing electrical impulses which occur during said intervening periods, means for superimposing said interfering signals and said impulses, I

said impulses having an amplitude and polarity such that said interfering signals are placed beyond a certain level which is beyond the peak amplitude of the picture signals, a signal channel so adjusted that it will not pass signals beyond said certain level, and means for supplying said superimposed signals and impulses to said signal channel whereby picture signals appear in the output of said channel to the exclusion of said interfering signals. p

RICHARD L. CAMPBELL. 

